Coordinate Compounds - Important Examples and Properties of Coordination Compounds

  • Definition: A coordination compound is a compound in which a central metal ion is surrounded by a group of ligands that are attached to the ion through coordinate covalent bonds.
  • Examples:
    • [Fe(CN)6]4-
    • [Cu(NH3)4]2+
    • [Co(NH3)6]3+
  • Properties of Coordination Compounds:
    • Coordination Number: The number of ligands attached to the central metal ion.
    • Ligand: An ion or molecule that donates a pair of electrons to the metal ion.
    • oxidation State: The charge of the metal ion in the complex.
    • Isomerism: Coordination compounds can exhibit different types of isomerism.
    • Color: Many coordination compounds are colored due to the presence of metal d-orbitals.
    • Magnetic Properties: Some coordination compounds are paramagnetic, while others are diamagnetic.

Coordination Number and Ligands

  • Coordination Number:
    • Refers to the number of ligands bonded to the central metal ion.
    • Examples:
      • [Cu(NH3)4]2+ (coordination number = 4)
      • [Co(NH3)6]3+ (coordination number = 6)
  • Ligands:
    • An ion or molecule that donates a pair of electrons to the metal ion.
    • Examples:
      • NH3 (ammonia)
      • CN- (cyanide)
      • H2O (water)
      • CO (carbon monoxide)
  • Ligand Types:
    • Monodentate: Ligands that donate only one electron pair.
    • Bidentate: Ligands that donate two electron pairs.
    • Polydentate: Ligands that donate multiple electron pairs.

Oxidation State

  • Oxidation State:
    • The charge of the metal ion in the coordination compound.
    • Examples:
      • [Fe(CN)6]4- (Fe: +2 oxidation state)
      • [Cu(NH3)4]2+ (Cu: +2 oxidation state)
      • [Co(NH3)6]3+ (Co: +3 oxidation state)
  • Oxidation Number vs. Coordination Number:
    • The oxidation state of the metal ion is not always equal to the coordination number.
    • Example: In [Cr(H2O)6]3+, the coordination number is 6, but the oxidation state of Cr is +3.

Isomerism in Coordination Compounds

  • Isomerism:
    • Coordination compounds can exhibit different types of isomerism.
    • Different arrangement of ligands around the central metal ion.
  • Types of Isomerism:
    • Structural Isomerism: Different connectivity between ligands and central metal ion.
    • Geometrical Isomerism: Different spatial arrangement of ligands.
    • Optical Isomerism: Different arrangement of ligands, resulting in non-superimposable mirror images.
  • Examples:
    • Structural Isomerism: [Co(NH3)5(NO2)]SO4 and [Co(NH3)5(SO4)]NO2
    • Geometrical Isomerism: [Pt(NH3)2Cl2] (cis) and [Pt(NH3)2Cl2] (trans)
    • Optical Isomerism: [Co(en)3]3+ (D-enantiomer) and [Co(en)3]3+ (L-enantiomer)

Color of Coordination Compounds

  • Color:
    • Many coordination compounds are colored due to the presence of metal d-orbitals.
    • Transition metals exhibit d-d electronic transitions.
  • Factors Affecting Color:
    • Nature of Metal Ion: Different metal ions have different d-orbital energies.
    • Nature of Ligands: Different ligands cause different energy changes in d-orbitals.
    • Crystal Field Splitting: The energy difference between d-orbitals in the presence of ligands.
  • Example:
    • The blue color of [Cu(H2O)6]2+ is due to the absorption of light in the red region, while the transmitted light appears blue.

Magnetic Properties

  • Magnetic Properties:
    • Some coordination compounds are paramagnetic, while others are diamagnetic.
    • Depends on the number of unpaired electrons on the metal ion.
  • Paramagnetic:
    • Coordination compounds with unpaired electrons are attracted to a magnetic field.
    • Example: [Fe(H2O)6]3+ (5 unpaired electrons)
  • Diamagnetic:
    • Coordination compounds with all electrons paired are not attracted to a magnetic field.
    • Example: [Co(NH3)6]3+ (all electrons paired)
  • Magnetic Behavior and Coordination Number:
    • Generally, higher coordination numbers tend to exhibit paramagnetic behavior.

Summary

  • Coordination compounds consist of a central metal ion and ligands bonded through coordinate covalent bonds.
  • Examples include [Fe(CN)6]4-, [Cu(NH3)4]2+, and [Co(NH3)6]3+.
  • Important properties include coordination number, ligands, oxidation state, isomerism, color, and magnetic properties.
  • Coordination compounds can exhibit different types of isomerism, such as structural, geometrical, and optical isomerism.
  • The color of coordination compounds is due to the presence of metal d-orbitals and factors like the nature of the metal ion, ligands, and crystal field splitting.
  • Magnetic properties depend on the presence of unpaired electrons on the metal ion. Paramagnetic compounds are attracted to a magnetic field, while diamagnetic compounds are not.
  1. Coordination Number and Ligands
  • Coordination Number:
    • Refers to the number of ligands bonded to the central metal ion.
    • Examples:
      • [Cu(NH3)4]2+ (coordination number = 4)
      • [Co(NH3)6]3+ (coordination number = 6)
  • Ligands:
    • An ion or molecule that donates a pair of electrons to the metal ion.
    • Examples:
      • NH3 (ammonia)
      • CN- (cyanide)
      • H2O (water)
      • CO (carbon monoxide)
  • Ligand Types:
    • Monodentate: Ligands that donate only one electron pair.
    • Bidentate: Ligands that donate two electron pairs.
    • Polydentate: Ligands that donate multiple electron pairs.
  1. Oxidation State
  • Oxidation State:
    • The charge of the metal ion in the coordination compound.
    • Examples:
      • [Fe(CN)6]4- (Fe: +2 oxidation state)
      • [Cu(NH3)4]2+ (Cu: +2 oxidation state)
      • [Co(NH3)6]3+ (Co: +3 oxidation state)
  • Oxidation Number vs. Coordination Number:
    • The oxidation state of the metal ion is not always equal to the coordination number.
    • Example: In [Cr(H2O)6]3+, the coordination number is 6, but the oxidation state of Cr is +3.
  1. Isomerism in Coordination Compounds
  • Isomerism:
    • Coordination compounds can exhibit different types of isomerism.
    • Different arrangement of ligands around the central metal ion.
  • Types of Isomerism:
    • Structural Isomerism: Different connectivity between ligands and central metal ion.
    • Geometrical Isomerism: Different spatial arrangement of ligands.
    • Optical Isomerism: Different arrangement of ligands, resulting in non-superimposable mirror images.
  • Examples:
    • Structural Isomerism: [Co(NH3)5(NO2)]SO4 and [Co(NH3)5(SO4)]NO2
    • Geometrical Isomerism: [Pt(NH3)2Cl2] (cis) and [Pt(NH3)2Cl2] (trans)
    • Optical Isomerism: [Co(en)3]3+ (D-enantiomer) and [Co(en)3]3+ (L-enantiomer)
  1. Color of Coordination Compounds
  • Color:
    • Many coordination compounds are colored due to the presence of metal d-orbitals.
    • Transition metals exhibit d-d electronic transitions.
  • Factors Affecting Color:
    • Nature of Metal Ion: Different metal ions have different d-orbital energies.
    • Nature of Ligands: Different ligands cause different energy changes in d-orbitals.
    • Crystal Field Splitting: The energy difference between d-orbitals in the presence of ligands.
  • Example:
    • The blue color of [Cu(H2O)6]2+ is due to the absorption of light in the red region, while the transmitted light appears blue.
  1. Magnetic Properties
  • Magnetic Properties:
    • Some coordination compounds are paramagnetic, while others are diamagnetic.
    • Depends on the number of unpaired electrons on the metal ion.
  • Paramagnetic:
    • Coordination compounds with unpaired electrons are attracted to a magnetic field.
    • Example: [Fe(H2O)6]3+ (5 unpaired electrons)
  • Diamagnetic:
    • Coordination compounds with all electrons paired are not attracted to a magnetic field.
    • Example: [Co(NH3)6]3+ (all electrons paired)
  1. Magnetic Behavior and Coordination Number:
  • Generally, higher coordination numbers tend to exhibit paramagnetic behavior.
  1. Hybridization in Coordination Compounds
  • Hybridization:
    • In coordination compounds, the central metal ion undergoes hybridization to form covalent bonds with ligands.
  • Types of Hybridization:
    • sp: Linear structure, two sigma bonds around the metal ion.
    • sp2: Trigonal planar structure, three sigma bonds around the metal ion.
    • sp3: Tetrahedral or octahedral structure, four or six sigma bonds around the metal ion.
  • Examples:
    • [Ag(NH3)2]+: sp hybridization (linear structure)
    • [Cu(CN)4]2-: sp3 hybridization (tetrahedral structure)
  1. Ligand Field Theory
  • Ligand Field Theory:
    • Explains the electronic structure and properties of coordination compounds based on the interaction between the metal ion and ligands.
  • Crystal Field Theory:
    • A simplified approach to understand the splitting of d-orbitals in coordination compounds due to the electric field of ligands.
  • Ligand Field Splitting:
    • Energy difference between the d-orbitals in the presence of ligands.
    • Determines the color and magnetic properties of coordination compounds.
  1. Stability of Coordination Compounds
  • Stability of Coordination Compounds:
    • Influenced by factors such as chelation, nature of ligands, and complex formation.
  • Chelation:
    • The ability of a ligand to donate multiple electron pairs to the metal ion.
    • Forms a more stable coordination complex.
  • Nature of Ligands:
    • Ligands with strong field strength tend to form more stable complexes.
    • Ligands with high electronegativity or multiple donor atoms typically have strong field strength.
  1. Nomenclature of Coordination Compounds
  • Nomenclature:
    • IUPAC system is used for naming coordination compounds.
  • Rules for Naming:
    • Name the ligands in alphabetical order before the metal ion.
    • Use prefixes to indicate the number of ligands.
    • Common ligands have specific names, such as aqua (H2O), cyano (CN-), and ammine (NH3).
  • Examples:
    • [Fe(CN)6]4-: Hexacyanoferrate(II) ion
    • [Cu(NH3)4]2+: Tetraamminecopper(II) ion "

Factors Affecting Complex Stability

  • Factors that influence the stability of coordination compounds:
    • Nature of ligands: Different ligands have varying abilities to stabilize the metal ion.
    • Charge on the metal ion: Higher charges on the metal ion increase stability.
    • Nature of the metal ion: Certain metal ions have a higher tendency to form stable complexes.
    • pH: The acidity or basicity of the solution affects the stability of metal-ligand bonds.
    • Temperature: Changes in temperature can influence the stability of coordination compounds.
  • Example:
    • The complex [Cu(NH3)4]2+ is more stable than [Cu(H2O)4]2+ due to the stronger bonding of NH3 ligands.

Ligand Exchange Reactions

  • Ligand exchange reactions:
    • Reactions where one or more ligands in a coordination complex are replaced by other ligands.
  • Ligand Substitution:
    • The process of exchanging one ligand for another in a coordination compound.
    • Example: [Cu(NH3)4]2+ + 2H2O ⇌ [Cu(H2O)4]2+ + 4NH3
  • Factors affecting ligand exchange reactions:
    • Nature of the metal ion: Some metal ions have a higher tendency to undergo ligand substitution.
    • Nature of the ligands: Ligands with stronger donor abilities can displace weaker ligands.
    • Reaction conditions: Temperature, pH, and concentration can influence the rate and extent of ligand substitution.

Stability Constant

  • Stability Constant (K):
    • Measure of the stability of a coordination compound with a specific ligand.
    • Indicates the equilibrium position of the ligand exchange reaction.
  • Formation Constant (Kf):
    • Measures the stability of a complex formed by adding ligands to the metal ion.
    • Larger Kf value indicates a more stable complex.
  • Dissociation Constant (Kd):
    • Measures the stability of a complex by dissociating a ligand from the metal ion.
    • Smaller Kd value indicates a more stable complex.
  • Example:
    • Kf for [Cu(NH3)4]2+ is larger than Kf for [Cu(H2O)4]2+, indicating the greater stability of the coordination complex with NH3 ligands.

Acid-Base Reactions of Complex Ions

  • Acid-Base Reactions:

    • Complex ions can act as either acids or bases in solution.

  • Acidic Complexes:

    • Complex ions that release H+ ions in solution.
    • Example: [Ni(H2O)6]2+ ⇌ [Ni(H2O)5OH]+ + H+

  • Basic Complexes:

    • Complex ions that accept H+ ions in solution.
    • Example: [Cu(NH3)4(H2O)]2+ + H+ ⇌ [Cu(NH3)4(H2O)2]3+

  • pH Dependence:

    • Acidic and basic properties of complex ions depend on the pH of the solution.

Chelation and Chelates

  • Chelation:
    • The formation of a complex ion using multidentate ligands (chelating agents).
    • Chelating agents have multiple donor atoms to form multiple bonds with a metal ion.
  • Chelating Agents:
    • Examples: ethylenediaminetetraacetic acid (EDTA), diethylenetriamine (dien), porphyrins.
    • Chelating agents enhance the stability of coordination compounds due to the formation of chelates.
  • Chelates:
    • Complex ions that involve multiple bonds between a metal ion and a multidentate ligand.
    • Chelates are more stable than complexes formed with monodentate ligands.

Applications of Coordination Compounds

  • Biological Significance:
    • Important role in biological processes, such as enzymes and transport functions.
    • Examples: Hemoglobin, chlorophyll, vitamin B12.
  • Medicinal Applications:
    • Coordination compounds used in medicine for therapeutic purposes.
    • Examples: Cisplatin (anticancer drug), Prussian blue (treatment for heavy metal poisoning).
  • Industrial Applications:
    • Coordination compounds used in various industries.
    • Examples: Catalysts, pigments, dyes.

Environmental Significance

  • Environmental Significance:
    • Coordination compounds play a crucial role in environmental processes and pollution control.
  • Environmental Applications:
    • Coordination compounds utilized in wastewater treatment, soil remediation, and pollutant monitoring.
    • Examples: Zeolites, chelating agents for metal ion removal.
  • Environmental Impacts:
    • Coordination compounds can have adverse effects on the environment.
    • Examples: Metal contamination from mining activities, bioaccumulation in aquatic organisms.

Summary

  • Factors affecting stability include nature of ligands, charge on metal ion, nature of metal ion, pH, and temperature.
  • Ligand exchange reactions involve the substitution of one or more ligands in a coordination compound.
  • Stability constants (Kf and Kd) measure the stability of coordination compounds.
  • Complex ions can act as acids or bases in solution, depending on their behavior towards H+ ions.
  • Chelation involves the formation of complex ions using multidentate ligands, leading to the formation of chelates.
  • Coordination compounds find applications in biology, medicine, industry, and environmental processes.